Method of removing scale from ferrous articles



Patented Sept. 25, 1951 METHOD OF REMOVING SCALE FROM FERROUS ARTICLES Charles B. Francis, Pittsburgh, Pa., assignor to United States Steel Company, a corporation of New Jersey No Drawing. Application February 21, 1946, Serial No. 649,380

3 Claims.

1 This invention relates to a method of cleaning ferrous bodies and to a bath therefor, and more particularly to the removal of scale or oxide from the surfaces of such bodies. The invention may be used to clean ferrous bodies in general, but it displays particular advantages in the cleaning and descaling of bodies of high chromium or high chromium-high nickel steels known as stainless, corrosion resistant, and heat resistant steels.

Among the objects of the invention is the overcoming of the difliculties and hazards entailed in the practice of prior art processes for cleaning ferrous bodies, particularly those containing appreciable amounts of chromium.

Another object of the invention is the provision of a cleaning method which markedly shortens the time and reduces the cost of such cleaning and descaling of ferrous bodies.

Yet another object of the invention is the provision of a simple, eificient, easily operated. method of cleaning ferrous bodies, particularly those of the above pointed out class or type of stainless, heat resistant, and corrosion resistant steels.

These and other objects of the invention will become more fully apparent in the following disclosure.

Whereas the cleaning and descaling process and bath therefor of the present invention are applicable to ferrous bodies generally, as will be more fully appreciated hereinafter, they display their maximum advantages in cleaning and removing scale from chromium 0r chromium-nickel irons and'steels, in which such process has heretofore been very expensive and time consuming. For purposes of illustration therefore, the invention will be described in connection with such irons and steels, and more particularly in connection with steels of such type which are more commonly used.

Whereas there are numerous types of each of the kinds of steel mentioned above, they may be classified, in general, in four classes:

1. Straight chromium steel;

2. Those containing chromium as the principal alloying element and oneor more elements such as silicon, molybdenum, copper, sulphur, selenium, titanium or tungsten added in smaller proportions to modify the properties of the straight chromium type;

3. Those that contain chromium and nickel as the principal alloying elements; and

4. Those that contain chromium and nickel as the principal alloying elements and one or more additional elements such as set out in type 2 above.

Ill

In general, these steels contain more than 4% chromium and are made in electric furnaces, their fabrication involving casting into ingots and subsequent conventional hot working, heat treating, and, usually, cold working operations, to produce the desired articles. During the hot working and heat treating operations the surfaces of all such steels become oxidized from exposure to furnace gases or air while at the elevated temperatures to which they are heated. In addition, the surfaces may gather other matter such as spots of oil or grease.

Because of their compositions the oxidation products formed on the surfaces of these steels during the various hot working and heat treating operations adhere tightly to the surfaces and are in general as resistant to chemical attack as the steels themselves. In the prior art the processes used to remove these surface oxidation products may be classified as mechanical and chemical, the former including such operations as sand or shot blasting, grinding and milling, and the latter including such processes as pickling, that is, treatment in hot solutions of acid, and treatment in fused baths.

Mechanical cleaning of such steels by sand or shot blasting is expensive and has the further objection that the surfaces are roughened and to some extent hardened, as in cold working. As a consequence, such mechanical cleaning methods for such metals have been practically abandoned.

Pickling also has been found to be unsatisfactory, since it-is very time consuming in that it requires rather long immersion in successive acid baths with intermediate washing and scrubbing operations. Furthermore, pickling does not altogether clean the steels, since it leaves their surfaces covered with smut, a black deposit that adheres tightly to the surfaces, which must be removed by mechanical scrubbing. In addition to the lack of efficiency in cleaning the steels, the pickling method is alsoopen to objection that spent pickling baths must be disposed of, a problem for which no entirely satisfactory solution has been found to date.

In view of the disadvantages of mechanical methods and acid pickling methods for the cleaning and descaling of such steels, attempts have been made and are still being made to utilize molteh baths for such purpose. such baths have been composed of sodium or potassium hydroxide or mixtures thereof, the articles to be cleaned being immersed in such baths to allow them to act upon the coatings. Hydroxide baths alone have been found not to act efa,sc9,ite I fectively upon the oxide coatings, and consetheir effectiveness through the addition of low fusing, strongly reducing, agents such as sodium hydride and sodium cyanide. Both the hydride and the cyanide in such molten baths react in the same manner, that is, they reduce the iron,

chromium, and nickel oxides on the surface to'a metallic state. Ordinarily the hydride is used in preference to.the cyanide to avoid the health hazard of the latter, as well as the danger of recarburizing the surface of the steel treated, which would decrease-its resistance to corrosion.

The use of the hydride, however, is itself not without explosion hazard since it is produced by adding metallic sodium to a boxed-in section of the molten hydroxide bath and bubbling hydrogen through the molten sodium. Furthermore, both the hydride and cyanide containing baths are slow and costly and have low cleaning efflciencies so that generally they must be supplemented by further pickling steps to clean the products completely.

The above discussed fused cleaning bath con- 1 taining cyanide or hydride operates, as has been mentioned, by reduction of the oxide or scale on the metal to be cleaned. Another proposed fused bath process of cleaning employs a sodium hydroxide bath to which sodium nitrate is added, such bath depending in its operation on the oxidation of the oxide or scale to facilitate its subsequent removal. Such latter bath will be more fully discussed hereinafter.

The cleaning and descaling method of the present invention comprises generally the contacting of the surfaces to be cleaned with a molten higher oxide of an alkali metal whereby the iron and chromium oxides are rapidly and effectively converted to higher oxides or other compounds which are readily soluble in water, and which thus may be easily subsequently removed by washing with water. In the preferred embodiment of the invention use is made of a fused alkali metal hydroxide bath, the higher oxide of an alkali metal being added thereto in minor proportions. The fused alkali metal hydroxide serves as a heating medium for the metal and a diluent for the molten higher oxide of an alkali metal, it being found that a relatively low concentration of the latter is effective and forms a convenient, economical bath for cleaning metals in accordance with the method of the invention.

Operation of the cleaning method will be described, -for purposes of illustration, in connection with the batch cleaning of sheets or plates of stainless steel. It will be apparent, however, that the method may be employed in connection with the continuous cleaning and descaling of ferrous sheet and strip products by traversing such products through a cleaning bath of the described composition. In either the batch or the continuous process the essential part of the equipment necessary, in the preferred embodiment, is the fused alkali metal hydroxide bath which, for practice of the batch process, is preferably made in the form of a large rectangular tank of the dimensions required to permit immersion of the articles in batches. For example,

to treat sheets in batches may require a deep are attacked but slowly by the ingredients of the bath. Alternatively, the tank may be made of lighter plates lined with heavy sheets of nickel,

nickel being highly resistant to chemical action by the bath, or with other corrosion resistant metal. The bath in such tank may be heated and maintained at the desired temperature in various manners, but it is preferred for efliciency to heat it internally as by immersing heating elements in the bath itself.

Although it is o'ssible to immerse the unheated articles to I: cleaned directly into the cleaning bath, especially if their mass is small relative to that of the bath, it is preferred that they be heated prior to such immersion, since such preheating saves considerable time in bringing the articles up to temperature in the descaling bath, avoids chilling the bath, and prevents solidification of the fused mixture upon the surfaces of the articles when they are immersed, thus decreasing the overall time of immersion to insure removal of the scale. Such preheating may be accomplished in any known heating chamber or furnace, which in either the batch or continuous processes will be interposed in the processing line between prior metal treating or working devices and the descaling bath.

In the preferred embodiment, the molten descaling bath consists predominantly of a fused mixture of sodium hydroxide and a minor amount of higher oxide of an alkali metal. By higher oxide is meant an oxide having a greater ratio of oxygen to metal than that existing in the lowest oxide, the monoxide. It is preferred that, in general, the temperature of the bath shall be below 900 F. and preferably between 700 and 900 F. Such temperature has been found sufliciently high to give fast, eflicient cleaning action but at the same time not high enough to cause undesirably fast decomposition of the bath or changes in the metal such as carbide precipitation or deleterious tempering effects. Any one of the hydroxides of the alkali metals, that is lithium, sodium. potassium, rubidium, and caesium may be employed as the alkali metal hydroxide in the present process. All such hydroxides have melting points below 900 F. and thus may be used in the preferred method where the bath is kept at or below 900 F. Be-

0 ,cause sodium hydroxide is cheap and easily available, it is preferred to use it rather than the other hydroxides. It will be further understood that mixtures of the above hydroxides may be employed if desired.

As the higher oxide of an alkali metal for the preferred cleaning bath, which is maintained at temperatures of 900 F. or below, such oxides should have melting points at or below the temperature at which the bath is maintained. For such baths sodium peroxide, NaaOz, potassium trioxide, K203, potassium peroxide, K02, rubidi-' um tetroxide, Rb204, caesium dioxide, C5202, and caesium trioxide, C5203, or mixtures thereof, may be employed. In those instances where maintenance of the bath above 900 F. is feasible, as for instance when the metal is not deleteriously affected by such higher temperature, other higher oxides of an alkali metal may be employed. Such other higher oxides are potassium dioxide,

'KaOzrubidium dioxide, RbzOz, rubidium trioxide, RbzOs, and caesium tetroxide, C5204, or mix-'- tures thereof. In all cases, of course, the bath must be maintained at a temperature sufficientto maintain it in its fluid fused state. In the bath which is ordinari y preferred for the treatthe fused alkali metal hydroxide and a minor amount of fused higher oxide of alkali metal which may range from .25% upwardly by weight of the bath. Whereas theoretically there is no upper limit to the higher oxide content. and a content of from to of such oxide would effect cleaning very efliciently, such relatively high higher oxide contents are impractical because of the cost of such oxides and their relatively fast breakdown. From the point of view of cost the practical upper limit of the higher oxide of an alkali metal content appears to be in the neighborhood of 3 /2%, and the usually preferred range of the oxide lies between .5% and 3% by weight of the bath. The choice of the exact oxide content employed will be governed by the type of finish desired, the type of metal being cleaned, and the time in which it is desired to effect the cleaning. Then too, the rate of attack upon the tank must be considered, since such rate increases as the concentration of the alkali metal oxide is increased. Regarding the finish obtained, this refers to the appearance of the surface after treatment with respect to its luster and varies from dull to bright, a sodium peroxide concentration of less than about 1% for instance giving a dull finish while higher concentrations give an increasingly bright finish, the ultimate degree of brightness obtainable depending largely upon the smoothness of the surface beneath the scale and oxide removed.

It is to be understood that whereas the hydroxide and thehigher oxide of an alkali metal constitute the main components of the bath, other components in minor amounts, such as the impurities usually found in commercial grades of the alkali metal hydroxides and higher oxides, do not interfere with the operation of the bath. Furthermore, small amounts of the alkali salts may be added to the bath, the proportion depending on the fusion point and composition of the salt. without adversely affecting the action of the bath. Then too, as will appear hereinafter, the bath contains increasing amounts of other materials, including the decomposition product of the higher oxide, the longer the bath is used. When'sodium peroxide is employed as the active agent, its decomposition product is sodium monoxide, NazO.

Assuming that a bath of sodium hydroxide and sodium peroxide in accordance with the above ranges has been made up and heated to the predetermined temperature between 700 and 900 F., and that a rack of stainless steel sheets has been heated to approximately the temperature of the bath, the cleaning and descaling operation proceeds as follows. The rack of heated sheets is immersed in the bath and permitted to remain therein until the oxide or scale has been dissolved from the surface. With such preheating of the sheets, the time of immersion may vary, for example. from 30 seconds to 12 or 15 minutes or more, the time depending upon the thickness of the scale to be removed, the condition of the surface of the sheets, that is whether pitted or smooth, the type of steel being treated and to some extent the temperature of the bath and the concentration of peroxide. Immersion for a longer time than necessary is not harmful to the steel treated provided it contains less than .5% silicon, but it will obviously affect the production adversely and will increase the consumption of sodium peroxide per unit area of surface cleaned, and thus is ordinarily to be avoided. with steels high in silicon long immersion in the bath after the scale has been removed may roughen or pit the surfaces. Thus, for each steel and scale condition the minimum time in the bath should be ascertained by trial or experience and adhered to as closely as possible.

At the end of the immersion period the sheets are removed from the bath and rinsed in water. This is preferably effected by lifting the sheets above the bath, holding them there while the excess liquid of the bath drains therefrom, and

then moving the rack of sheets to immerse them in a tank of water where they are allowed to remain for a few seconds to remove the film of hydroxide from the bath adhering to the surfaces. This wash and rinse leaves the surfaces of the. sheets coated with a film of red iron hydroxide which is precipitated by the reaction of the wash a water with sodium ferrite on the sheets, the sodium ferrite resulting from the conversion of the oxide and scale on the sheets by the cleaning bath.

The red iron hydroxide coating may usually be removed by dipping the sheets in a bath of warm dilute hydrochloric acid, but since this acid rapidly attacks many of the steels of this class, its use is ordinarily to be avoided. It is preferred instead to dip the sheets in a hot (140 F. or higher) aqueous solution of sulphuric acid, which generally leaves the surface darkened with a smut that may be removed mechanically as by scrubbing in the presence of a liquid such as water. Instead of scrubbing, however, it is ordinarily preferred to remove it by dipping the sheets into a hot (120 F. or higher) dilute aqueous solution of nitric acid, of 2% and higher concentration, containing .05% or more of hydrofluoric acid, which leaves the surfaces with a metallic luster, the smoothness, of

- course, being predetermined by the previous mechanical treatments such as rolling. In the treatment of certain grades of steel such as 18-8 chrome-nickel steels, the sulphuric acid dip may be omitted and the brown to reddish deposit removed by a single dip in nitric acid or nitric and hydrofluoric acid. To remove the acid the sheets are washed by dipping them in water, after which they are dried to give a finish acceptable to the trade or adapted to any succeeding operations to be desired.

It is necessary. when the bath is used in the cleaning and descaling of successive batches of sheets or in the continuous cleaning of sheet and strip material, to retain the bath within the desired concentration of the higher oxide of the alkali metal, in this case sodium peroxide. In the batch cleaning of sheets it is convenient to make additions of the higher oxide just before each batch of sheets is immersed, thereby insuring the correct composition of the bath for treating such batch. Such procedure is preferred because the higher oxides, such as sodium peroxide,

undergo slow thermal decomposition so that in time the bath becomes depleted of this reagent whether steel is being cleaned or not. The concentration of alkali metal hydroxide in the bath may be easily and quickly determined by a relatively simple chemical test, and the same is true of the monoxide of the alkali metal. From the resuits of such test. it can easily be determined how 7 much of the higher oxide should be added to the bath at any time during the cleaning operation, as well as just before a fresh batch of sheets is immersed.

No decomposition of the alkali metal hydroxide occurs, but there is a. loss of this compound because of the carry-out of the bath composition on the articles as they are withdrawn. Thus, if a bath employing sodium hydroxide and sodium peroxide were operated at high concentrations of sodium peroxide with no additions of sodium hydroxide, it might eventually be composed of sodium monoxide, which is the decomposition product of the sodium peroxide, and sodium peroxide. It is obvious, therefore that to maintain the bath at substantially the same composition which it has initially, sodium hydroxide should be added occasionally. It will be apparent that after use of the bath in the cleaning operation it will contain, in addition to the initial and regularly added ingredients, a certain amount of the reaction products of the oxide or scale on the articles and the components of the bath. These reaction products, in the case of chromium containing steels, as

will be more apparent hereinafter, are mainly.

sodium salts of ferric and chromic acids held either in solution or suspension. The change in composition of the bath occasioned by the forming of such reaction products has no marked effect upon the efficiency or the fusing temperature of the bath.

After the bath has been used a while for cleaning, the products formed by reaction of the sodium peroxide with the scale are precipitated and settle to the bottom of the bath and it becomes necessary eventually to remove the sediment. Removal of the sediment may be accomplished in various ways, a convenient method being to cover the bottom of the tank with a number of shallow rectangular pans equipped with channels on the ends so that they may be lifted out of the tank, as by means of grab hooks, as they become filled.

Although it is to be understood that the method of cleaning and descaling ferrous articles and the bath therefor of the present invention are not to be confined to the'following theory of operation, experimental evidence points stongly to the correctness of such theory. From observed phenomena it appears that the chief function of the alkali metal hydroxide and of the alkali metal monoxide subsequently formed as a decomposi tion product or a reaction product of the higher oxide of an alkali metal is to serve as a diluent for the higher oxide which is the reagent active in removing oxide in scale.

While the protective oxide film formed on the polished surfaces of steels of the stainless and heat resisting types under consideration consists mainly of chromic oxide (ClzOa) intimately mixed with a smaller proportion of ferric oxide (FezOa) the comparatively thick coating of scales or oxides formed at high temperatures, and particularly when the steels are exposed to the heating furnace atmospheres, are more complex in character and vary in composition from the outer surface of the scale or oxide down to the surface of the metal. At the outer surface are found the oxides of iron, R304 and Pet), and the oxide of chromium, CrzOa, on steels of the straight chromium types.

with the probability that some of the FeO may be combined with CrzO; as chromite FeO-CrQOa and a little with silica to form iron silicate, if the steel contains silicon. On steels of the chromiumnickel type, nickel oxide, NiO, may be formed, intermingling with the other oxides. At a distance beneath the surface the oxides exist mainly as FeO and CrzOa, and if the steel contains nickel, it will be present as the metal, not the oxide. Lying adjacent to the surface of the metal will be found the lower oxide of chromium, CrO, intermingled with FeO and metallic iron or metallic iron and metallic nickel.

Since all these oxides are bases, none of them will react with either the alkali metal monoxide or the alkali metal hydroxide. In the presence of a molten higher oxide of an alkali metal, taken here as the preferred sodium peroxide, the iron and chromium oxides are oxidized to ferric oxide, F8203, and chomium trioxide, CrO respectively. which are acids that combine readily with either sodium monoxide, NazO, or the alkali metal hydroxide, such as sodium hydroxide. The following reactions show the effect of sodium per.- oxide upon oxides of iron and chromium found in scales on the above types of steels:

(1); 2FeO+NazO2 NazO-FezOa or [q Na2F6204 (sodium ferrate) (I!) CI203+3N8.202 NazO+2NazCrOi (sodium chromate) The thin coating of ferric oxide or of ferric oxide and nickel oxide that remains on the surface is removed by dipping in hot dilute sulphuric acid followed, if necessary, by a dip in hot dilute nitric.

acid, as above explained.

It will be apparent from the above that the bath of the present invention may be used to clean ferrous articles such as plain mild steels if desired, although it would not normally be economically feasible to do so. In this case the above reaction 1 would take place to remove the coating of iron oxide from the steel. In the case of steels containing an appreciable amount of silicon, the silica existing in the scale when subjected to the bath is converted by a reaction with both the sodium oxide and the sodium hydroxide to sodium silicate in both cases, the silica bein'g acid and readily combining with sodium hydroxide, thus being readily removed from the article.

The bath and its method of use of the present invention display marked advantages over the proposed prior fused oxidizing bath composed of sodium hydroxide and sodium nitrate. In such proposed bath the sodium nitrate acts only as an oxidizing agent to form sodium nitrite which is not effective as a neutralizing agent, and which accumulates as a useless ingredient in the bath. In such bath, therefore, sodium hydroxide must serve as an agent to neutralize the acid oxides of iron and chromium. The sodium hydroxidesodium nitrate bath, in order to clean metal effectively, must be operated at a higher temperature than the preferred baths of my invention which are operated, as explained, at between 700 and 900 R, such higher temperature promoting thermal decomposition of the nitrate which proceeds whether metal is being cleaned or not. The

build-up of nitrite in such bath progresses with comparative rapidity to a certain concentration, beyond which it becomes necessary to feed both nitrate and hydroxide to the bath. Addition of the hydroxide is necessary to stabilize the nitrite, which otherwise undergoes thermal decomposition with the formation of toxic gaseous nitric oxides and sodium oxide until equilibrium is reached. Unless the carry-out of the bath material is considerable, therefore, these conditions require that the sodium hydroxide-sodium nitrate bath be dumped at intervals. In the bath of the present invention such build-up of inactive ingredients does not occur.

The method of cleaning and descaling ferrous metal and the bath therefor of the present invention thus show great advantages over the prior art fused bath methods of cleaning such metals. The cleaning may be eifected in a shorter time with less labor and a lower consumption of acids than in any of the processes of the prior art. In the preferred modification, wherein the bath is maintained at 900 F. or lower, it is advantageous in that it does not produce deleterious changes of properties in the metal being cleaned. The method is applicable with the same effectiveness to both hot rolled and annealed stock, and, because of the ease with which the scale is removed from'the metal it eliminates the necessity of closely controlling the atmospheres in the heat treating cycles.

Having thus fully described the cleaning and 10 descaling method and bath therefor of the present invention, I claim as new the following.

I claim:

1. In a method of descaling articles of stainless steel, the steps including immersing the articles in a bath of fused sodium hydroxide, pickling the articles in a dilute aqueous solution of sulphuric acid, and then dipping them in a dilute'aqueous solution of nitric acid.

2. The method as defined by claim 1 characterized by said nitric-acid solution containing a small amount of hydrofluoric acid.

3. The method as defined by claim 1 characterized by said bath containing from .5 to 3% sodium peroxide.

' CHARLES B. FRANCIS.

REFERENCES CITED The following references are of record in the OTHER REFERENCES Publication, Ephraim Inorganic Chemistry" 1943, 4th ed.

Publication, Lange Handbook of Chemistry." 

1. IN A METHOD OF DESCALING ARTICLES OF STAINLESS STEEL, THE STEPS INCLUDING IMMERSING THE ARTICLES IN A BATH OF FUSED SODIUM HYDROXIDE, PICKLING THE ARTICLES IN A DILUTE AQUEOUS SOLUTION OF SULPHURIC ACID, AND THEN DIPPING THEM IN A DILUTE AQUEOUS SOLUTION OF NITRIC ACID. 